Image display device

ABSTRACT

An image display comprises: a display device having an image output surface at which an image is displayed as a spaced array of pixel elements; an image guide ( 20 ) coupled to the image output surface of the display device and comprising a plurality of light transmission guides ( 80 ) each having an input end and an output end, the input ends of the light transmission guides being arranged relative to one another so that groups of one or more light transmission guides receive light from respective groups of one or for a cluster comprising at least a subset of the light transmission guides ( 80 ): at the outer periphery of the cluster, the input ends of the light transmission guides are constrained against expansion by a frame formed of a material having thermal expansion properties which are substantially similar to the thermal expansion properties of the image output surface.

This application is a national phase of International Application No.PCT/GB03/01969 filed May 8, 2003 and published in the English language.

This invention relates to displays.

The technology behind flat-panel displays, such as liquid crystal orplasma displays, has advanced to the stage where a single display can beeconomically manufactured to about the screen size of a modest domestictelevision set. To increase the display size of a single-unit displaybeyond this level introduces greater costs, lower manufacturing yieldsand other significant technical problems.

To provide larger displays, therefore, a hybrid technology has beendeveloped whereby multiple smaller rectangular displays are tessellatedto form the required overall size. For example, a 2×2 tessellated arrayof 15 inch diagonal displays, with appropriate addressing electronics toroute pixel information to the appropriate sub-display, would provide a30 inch diagonal display.

A drawback of this type of arrangement is that the active area of anindividual display, that is to say, the area of the front face of thedisplay on which pixel information is displayed, does not extend to thevery edge of the physical area of the display. The technologies used,whether plasma, liquid crystal or other, require a small border aroundthe edge of the active display area to provide interconnections to theindividual pixel elements and to seal the rear to the front substrate.This border can be as small as a few millimetres, but still causesunsightly dark bands across a tessellated display.

Various solutions have been proposed to this problem, most of which relyon bulk optic or fibre optic image guides to translate or expand theimage generated at the active area of the individual sub-displays.

For example, U.S. Pat. No. 4,139,261 (Hilsum) uses a wedge structureimage guide formed of a bundle of optical fibres to expand the imagegenerated by a panel display so that by abutting the expanded images,the gap between two adjacent panels, formed of the two panels' borderregions, is not visible. The input end of each fibre is the same size orless than a pixel element. The optical fibres are aligned, at theirinput ends, with individual pixel elements of the panel display, so thatthe pixel structure of the display is carried over to the output planeof the image expander. Other image guides formed in this way maytranslate the image to provide a border-less abutment between a pair ofadjacent panels. Various types of light transmission guide may be used,such as rigid or semi-rigid light transmission guides. It has beenproposed that the image guides should be fabricated from polymermaterials, for ease of manufacture.

In order to allow the input of an image guide to be aligned correctlywith a large array of pixel elements on a panel display, it is necessarythat the input ends of the light transmission guides are maintained inthe correct relative positions, often as a rectangular array of pixelpositions. Whatever means is used for registering the input ends of thelight transmission guides in their correct relative positions, a problemcan arise when the temperature of the display arrangement changes.

Most commonly, panel displays are fabricated of glass or a closelyrelated material. The surface of the panel display may have a thin layeror film of another material coupled (e.g. adhered) to it (for instancepolarising filters for a Liquid Crystal (LC) display). This layer mayhave different thermal expansion properties to those of the underlyingsurface. Of course the skilled person will understand that where this isthe case, the layer will expand (e.g. stretch or compress) with theunderlying surface when the underlying surface expands in response totemperature variations. Image guides proposed so far tend to use polymermaterials for the light transmission guides and/or for an arrangement(if one is used) for registering the input ends of the lighttransmission guides in the correct relative positions. A problemtherefore arises because the thermal expansion properties of glass andpolymer materials are different.

Consider an example display where the different thermal expansions ofthe input end of the image guide and of the display substrate mean thatthe image guide has expanded across its width by one pixel-width morethan the panel substrate. This could have two major effects on the imagedisplayed at the output of the image guide.

The first is a change to the spatial resolution of the display. Atpositions within the display area, some light transmission guides couldbe receiving substantially equal amounts of light from two adjacentpixel elements. This has a low-pass spatial filtering effect on thedisplayed image, and this effect will vary across the display area.

The second is that the outermost light transmission guides will bereceiving no light at all, as they will have expanded beyond the displayarea of the panel. This will cause an unsightly dark line at the outputof the image guides.

Viewed from a first aspect this invention provides an image displaycomprising:

-   -   a display device having an image output surface at which an        image is displayed as a spaced array of pixel elements;    -   an image guide coupled to the image output surface of the        display device and comprising a plurality of light transmission        guides each having an input end and an output end, the input        ends of the light transmission guides being arranged relative to        one another so that groups of one or more light transmission        guides receive light from respective groups of one or more pixel        elements; in which, for a cluster comprising at least a subset        of the light transmission guides:    -   at the outer periphery of the cluster, the input ends of the        light transmission guides are constrained against expansion by a        frame formed of a material having thermal expansion properties        which are substantially similar to the thermal expansion        properties of the image output surface.

In this aspect, the invention provides a physical constraint against theexpansion of the input end of the image guide (or a part of it) beyondthe extent defined by a frame which should have expansion propertieswhich are generally similar to those of the image output surface. So, asthe image output surface expands, so the frame should expand. If theinput end of the image guide has a tendency to expand more than this,the extra expansion is taken up by the compressible coupling between thelight transmission guides.

Although they could be loose from one another, for example only beingattached to the image output surface, it is preferred that the inputends of light transmission guides within the cluster are coupled to oneanother by a compressible coupling such as a compressible adhesive.

In order to approximate the expansion properties of a typical glassdisplay, it is preferred that the frame is formed of a glass or metalmaterial.

Preferably the cluster comprises substantially all (e.g. all) of thelight transmission guides.

Viewed from a second aspect this invention also provides an imagedisplay comprising:

-   -   a display device having an image output surface at which an        image is displayed as a spaced array of pixel elements;    -   an image guide coupled to the image output surface of the        display device and comprising a plurality of light transmission        guides each having an input end and an output end, the input        ends of the light transmission guides being arranged relative to        one another so that each light transmission guide receives light        from respective groups of one or more pixel elements; in which:    -   the input ends of the light transmission guides are individually        coupled to respective areas on the image output surface; and    -   the input ends of the light transmission guides are not coupled        to one another along a predetermined distance measured from the        input ends.

The invention addresses the problems described above by allowing somefreedom of movement (or rather, bending) of the input ends of the lighttransmission guides. This is achieved by not joining the input endstogether of a portion of their length, but joining them individually tothe image output surface of the display. So, if there is a slightdifferential expansion causing relative movement between the imageoutput surface and the input ends of the light transmission guides, thisis accommodated by a slight distortion of the uncoupled lengths of thelight transmission guides.

Viewed from a third aspect this invention also provides an image displaycomprising:

-   -   a display device having an image output surface at which an        image is displayed as a spaced array of pixel elements over an        active pixel region of the image output surface;    -   an image guide coupled to the active pixel region of the image        output surface and comprising a plurality of light transmission        guides each having an input end and an output end, the input        ends of the light transmission guides being arranged relative to        one another so that groups of one or more light transmission        guides receive light from respective groups of one or more pixel        elements; in which:    -   the image output surface has further pixel elements disposed        around the periphery of the active pixel region; and    -   the image guide can expand thermally so that the input of the        image guide encompasses the further pixels.

In this aspect, the invention addresses the problems described above byproviding, in effect, extra pixels at the outer periphery of the activepixel region. So, if there is a differential expansion causing the inputof the image guide to expand beyond the extent of the active pixelregion, there is still some light launched into the outermost lighttransmission guides of the image guide. This avoids the unsightly blackline referred to above.

To allow for a typical level of differential expansion, it is preferredthat the image output surface has further pixel elements disposed aroundthe periphery of the active pixel region over a guard band regionnarrower than the input end of a light transmission guide.

Preferably each light transmission guide is arranged to receive lightfrom two or more pixel elements.

To further reduce the visibility of an expansion of the image guidebeyond the active pixel region, it is preferred that the further pixelsare arranged to display substantially the same picture information asnearby (e.g. adjacent) pixels within the active pixel region or,alternatively (in a tiled or similar system), to display duplicateinformation to that of the peripheral pixels within the active pixelregion of an adjacent display.

Preferably the light transmission guides are coupled to the image outputsurface using an adhesive, although they could alternatively be held inplace by a mechanical arrangement such as a clip.

Although the invention is suitable for use with display devices such ascathode ray tube devices, it is preferred that the display device is apanel display device such as a liquid crystal panel display device.

The invention is particularly suitable for use when the image outputsurface is formed of a glass material and/or the light transmissionguides are formed of a polymer or plastics material.

The invention also provides an array of image displays as defined above,arranged so that viewing surfaces formed by the output ends of the imageguides abut to form a larger composite viewing surface.

Viewed from a fourth aspect this invention also provides an imagedisplay comprising:

-   -   a display device having an image output surface at which an        image is displayed as a spaced array of pixel elements over a        plurality of separated active pixel regions of the image output        surface;    -   an image guide coupled to the active pixel regions of the image        output surface and comprising a plurality of light transmission        guides each having an input end and an output end, the input        ends of the light transmission guides being arranged relative to        one another so that groups of one or more light transmission        guides receive light from respective groups of one or more pixel        elements and the output ends of the light transmission guides        forming a contiguous output surface; in which:    -   the input ends of the light transmission guides are arranged        into a plurality of sections, each section being associated with        a different one of the active pixel regions of the image output        surface, the light transmission guides in each section being        coupled to the active pixel region associated with that section.

In this aspect, the invention provides gaps between active regions onthe image output surface. In a preferred embodiment, the image guide isable to expand into this gap, reducing the degree of misregistrationoccurring under conditions of thermal expansion of the image guide. Inan alternative preferred embodiment, there is provided a registrationaid having thermal expansion properties substantially similar to thoseof the structure determining the expansion of the image output surface,to which the image guide is abutted. The registration aid is providedbetween the active areas of the image output surface and acts tominimise misregistration.

Viewed from a fifth aspect this invention also provides an image displaycomprising:

-   -   a display device having an image output surface at which an        image is displayed as a spaced array of pixel elements over an        active pixel region of the image output surface;    -   an image guide coupled to the active pixel region of the image        output surface and comprising a plurality of light transmission        guides each having an input end and an output end, the input        ends of the light transmission guides being arranged relative to        one another so that groups of one or more light transmission        guides receive light from respective groups of one or more pixel        elements, the light transmission guides being coupled together        along at least a portion of their length; in which:    -   the image guide is coupled to the image output surface over less        than the whole active pixel region of the image output surface,        the image guide being coupled to the image output surface such        that the image guide can expand with respect to the area        coupled.

In this aspect the invention aims to reduce mechanical distortion in,for instance, an adhesive layer coupling the image output surface andthe image guide together. The reduction in mechanical distortion duringvariations in temperature reduces the presence of adverse effects inimage quality and the probability of failure of the adhesive bond. Inone preferred embodiment, reference points are provided to preventrotational displacement of the image guide with respect to the imageoutput surface. In an alternative preferred embodiment, a plurality ofseparate active regions are provided on the image output surface, witheach of a number of groups of light transmission guides being attachedto an active regions. With this arrangement, rotational displacement iscontrolled because the image guide as a whole is attached to the imageoutput surface at a plurality of locations.

Various other respective aspects and features of the invention aredefined in the appended claims. Features from the dependent claims maybe combined with features of the independent claims as appropriate andnot merely as explicitly set out in the claims.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 is a schematic isometric rear view of a tiled array of displaypanels;

FIG. 2 is a schematic isometric front view of the array of FIG. 1;

FIG. 3 is a schematic side view of a display comprising a light source,a collimator/homogeniser, a display panel and an image guide;

FIG. 4 is a schematic side elevation of an array of light transmissionguides coupled to an image output surface, in accordance with a firstembodiment of the present invention;

FIG. 5 is a schematic side elevation of an array of light transmissionguides coupled to an image output surface, in accordance with a secondembodiment of the present invention;

FIG. 6 is a schematic plan view at a level A-A during the assembly of anarray as shown in FIG. 5;

FIG. 7 schematically illustrates the image output surface of a displaypanel having an active pixel region and a guard band;

FIG. 8 is a schematic side elevation of an array of light transmissionguides coupled to the image output surface of FIG. 7, in accordance witha third embodiment of the present invention;

FIG. 9 is a schematic plan view of the arrangement of FIG. 8;

FIG. 10 is a schematic plan view of an array of light transmissionguides coupled to an image output surface, in accordance with a fourthembodiment of the present invention;

FIG. 11 is a schematic side elevation of the arrangement of FIG. 10;

FIGS. 12 a and 12 b schematically illustrate a side elevation of a lightguide and a plan view of an image output surface in accordance with thefirst to fourth embodiments of the present invention;

FIGS. 12 c to 12 e are a schematic side elevation of a light guide andschematic plan views of an image output surface in accordance with afifth embodiment of the present invention;

FIGS. 13 a to 13 d are schematic plan views of a selection ofregistration aids applicable to the arrangement of FIG. 12 e;

FIG. 14 is a schematic plan view of an image output surface and theinput ends of a light guide in accordance with a sixth embodiment of theinvention;

FIG. 15 is a schematic plan view of an image output surface and theinput ends of a light guide (split into a plurality of groups) inaccordance with a seventh embodiment of the invention;

FIG. 16 a is a schematic plan view of an image output surface andregistration aid in accordance with an eighth embodiment of theinvention;

FIG. 16 b is a schematic plan view of an image output surface andregistration aid in accordance with a ninth embodiment of the invention;

FIGS. 17 a and 17 b schematically illustrate two alternative modes ofoperation of a guard band in accordance with embodiments of theinvention; and

FIG. 18 schematically illustrates tiled display control circuitry inaccordance with an embodiment of the invention.

FIG. 1 is a schematic isometric rear view of a tiled array of displaypanels.

The array comprises four display panels in a horizontal direction andthree display panels in a vertical direction. Each display panelcomprises a light emitting surface 10 and an image guide 20.

The light emitting surfaces 10 are each arranged as a plurality ofpixels or picture elements. In practice, they would include, forexample, a back light arrangement, focusing, concentrating and/orcollimating and/or homogenising optics and a liquid crystal panel or thelike, but much of this has been omitted for clarity of the diagram.

The panels each display portions of an overall image to be displayed.The portions represent adjacent tiles in a tessellated arrangement.However, because of the need to run electrical connections and physicalsupport around the edge of the light emitting surfaces 10, they cannotbe directly abutted without leaving a dark band or “black matrix” inbetween. So, the light guides 20 are used to increase the size of theimage from each light emitting surface 10 so that the output surfaces ofthe light guides 20 can be abutted to form a continuous viewing plane.

This arrangement is shown in FIG. 2 which is a schematic isometric frontview of the array of FIG. 1. Here, the output surfaces of the lightguides 20 abut so as to form a substantially continuous viewing surface30.

FIG. 3 is a schematic side view of a display comprising a light source40, a collimator/homogeniser 50, a liquid crystal panel 60 and a lightguide 70.

The light source 40 and the homogeniser 50 are shown in highly schematicform but in general terms are arranged to provide the back lightrequired by the liquid crystal panel 60.

The liquid crystal panel 60 may be of a type which uses a white or othervisible colour back light and provides liquid crystal picture elementsto modulate that back light for that display. Alternatively, the liquidcrystal panel 60 may be a photo luminescent panel which employs anultra-violet back light and modulates the ultra-violet light onto anarray of phosphors to generate visible light for display. Of course,many other types of light emitting surface 10 may be used such as anorganic light emitting diode array or even a cathode ray tube display. Atypical example of the surface material of the display might be CorningLCD Glass.

The image guide 70 comprises an array of light transmission guides 80,each of which carries light from a particular area on the liquid crystalpanel 60 to a corresponding particular area on an output surface 90. Indoing so, the light transmission guides are arranged to diverge so thatthe area covered on the output surface 90 is physically larger than theimage display area on the liquid crystal panel 60. This, as describedabove, allows an array of displays as shown in FIG. 3 to be abuttedwithout an unsightly black matrix at the viewing plane.

Some examples of materials used to fabricate the light transmissionguides are Bayer Makrolon Polycarbonate and Dow Caliber Polycarbonate.

FIG. 4 is a schematic side elevation of an array of light transmissionguides 80′ forming part of the image guide 20. The light transmissionguides 80′ are coupled, for example glued by a transparent adhesive, topixel elements 100 of a display panel 60′. In the example shown in FIG.4, each light transmission guide 80′ is coupled to a respectiveindividual pixel element 100. However, in this and other embodiments tobe described, each light transmission guide could be coupled to severalpixels (for example, a group of one or more each of red, green and bluepixels) or each pixel could be coupled to several light transmissionguides.

It will be appreciated throughout this description that the term“coupled to a pixel element” is to be understood in the context of apanel or other display. In the strictest sense, in for example a liquidcrystal display, the “pixel element” could be considered to be actuallywithin the sandwich structure of the various layers constituting thedisplay. However, using the term in an engineering sense, the skilledperson will of course understand that the light transmission guides arein fact coupled to positions on the outer surface of the display whichreceive light from the pixel element.

Examples of suitable transparent adhesives for this purpose are asfollows:

Epoxy Technology Epotech OG134 Hughes Associates Epoxy 330 Norland NOA61

Over a portion 110 of the length of the light transmission guides 80′,the light transmission guides are separate from one another. The gaps120 between the light transmission guides may be filled by air,near-vacuum, an inert gas, or even a low density flexible filler such asa silicone elastomer.

This arrangement allows the input ends of the light transmission guides80′ to track any differential expansion between the image guide 20 andthe image output surface of the display 60′ by a very slight distortionsuch as a bending or buckling at the input end. So, even if the imageoutput surface 60′ and the image guide expand laterally by differentamounts, each light transmission guide remains registered and in contactwith the correct one of the pixel elements 100.

The length of the unjoined region 110 depends on the dimensions of thelight transmission guides, their flexibility, and the strength of thebond between the light transmission guides 80′ and the pixel elements100 on the image output surface 60′. The range of temperatures overwhich the performance of the display is specified is also relevant. Theskilled person may establish an appropriate length of the region 110 byroutine experiment once these parameters are established.

FIG. 5 is a schematic side elevation of an array of light transmissionguides 80″ coupled to an image output surface 60″ in accordance with asecond embodiment of the present invention.

As before, each light transmission guide 80″ is coupled to a respectiveindividual pixel element 100″ but other arrangements are of coursepossible.

The light transmission guides 80″ are substantially independent overmuch of their length, being (for example) discrete optical fibres.However, a support 130 is provided along the length of the lighttransmission guides 80″, partly to help support the weight of the lighttransmission guides and also to assist in aligning each lighttransmission guide to the appropriate place on the image output surface60″ during assembly of the display arrangement. The position of thesupport 130 defines a distance 110″ over which the light transmissionguides are not coupled to one another. Again, this distance allows fordifferential thermal expansion between the image output surface 60″ andthe image guide.

If substantially flexible light transmission guides 80″ are used, thenin the case of a display having a large number of pixel elements 100″,it is useful to provide a technique for assembling the input ends of thelight transmission guides 80″ to the correct place for gluing on to theimage output surface 60″. FIG. 6 schematically illustrates such atechnique and is a schematic plan view taken along a section A-A of FIG.5.

Referring to FIG. 6, each light transmission guide 80″ is held in placelaterally by crossed sets of wires or blades 140. The crossed sets ofwires or blades define (in this example) square apertures in which eachlight transmission guide 80″ sits during the process of gluing the lighttransmission guide on to the appropriate pixel position on the imageoutput surface 60″.

The wires or blades 140 are supported at one end on a support member150. This holds them at the correct spacing for the pixel elements ofthe display panel in use. If fine blades are used, these may beself-supporting along their length and so no support is needed at thedistal end of each blade. If some distal support is needed, then theblades may be held by a clamp or an electromagnetic support to allow foran easy release.

So, the two sets of blades 140 are inserted in orthogonal directionsthrough the array of light transmission guides 80″ close to the surfaceof the panel display. If necessary, the blades are held at their distalends to maintain the correct spacing and to give structural rigidityduring the gluing process. An adhesive is applied to the surface of thepanel display and the image output surface 60″ is offered up to thearray of light transmission guides 80″. An image may be displayed on theimage output surface 60″ during this process to assist in obtaining thecorrect lateral positioning. Once the image output surface 60″ is in thecorrect position with respect to the array of light transmission guides80″, the adhesive is cured, for example by exposure to ultra-violetlight, by heat or simply by time elapsing. Then, if any support was usedat the distal ends of the blades 140, the support is released and theblades are carefully withdrawn along their length.

If flexible wires are used instead of rigid or semi-rigid blades, sometension is required between the support member 150 and an arrangementused to grip the wires at their distal ends, in order to maintain thecorrect spacing along the length of the wires. In this case, the distalend of each wire may be passed into a respective spaced groove in afurther support member (not shown), clamped in place and then pulled toplace the wire under tension. The process would then continue asdescribed above, but at the end of the assembly the tension would bereleased and the wires allowed to leave the respective grooves.

A third embodiment of the invention will now be described. FIG. 7schematically illustrates the image output surface 60′″ of a displaypanel having an active pixel region 170 which contains all of the pixelsneeded for an appropriate connection to an image guide, and a so-called“guard band” 180 formed of extra pixels disposed around the peripheraledge of the active pixel region 170.

The guard band provides additional pixels to give some light input intothe very outer light transmission guides of the image guide, in the casethat the image guide expands laterally beyond the active pixel region170. If this does happen, then the precise registration between eachlight transmission guide and the respective one or more pixel elementson the image output surface will be lost, and this could lead to someundesirable spatial low-passed filtering across the image. However, byproviding the guard band 180 having additional pixel elements, at leastan unsightly dark line around the edge of the image guide is avoided.

In this embodiment, pixels in the guard band 180 display the same colourand luminance as the nearest adjacent pixels in the active pixel region170. So, some pixel information is duplicated around the edge of thedisplay, but again this is less undesirable than an unsightly dark bandaround the output of the image guide.

In other embodiments, the pixels in the guard band 180 display the samecolour and luminance as the nearest adjacent pixels in the next adjacentdisplay in a tiled array of displays as shown in FIG. 1.

Two alternative modes of operation of the guard bands of a display willnow be described with reference to FIGS. 17 a and 17 b. FIG. 17 a is aschematic illustration of a first mode of operation of the guard bandsof a tiled display comprising four tiles. Each tile comprises an activearea 170 a, b, c and d and a guard band area 180 a, b, c and d. In thismode of operation, pixels 520 a, b, c and d in a given guard band 180 a,b, c or d will display the same information as the nearest neighbouringpixels in the active areas 170 a, b, c or d of a neighbouring tile. Forinstance, the guard pixel 520 a which lays on the edge of the activearea 170 d will display the same information as the pixel 510 a in theactive area 170 c. Similarly, the guard pixel 520 c and the guard pixel520 d (laying on the edge of active areas 170 b and 170 d respectively)will display the same information as the pixels 510 c and 510 drespectively (these pixels being part of the active areas 170 d and 170b respectively). In the case of the guard pixel 520 b, which lays on thecorner of the active area 170 a, the guard pixel 520 b will display thesame information as the corner pixel 510 b of the diagonally adjacentactive area 170 c. This is in contrast to the previous cases where theinformation is taken from an active area either horizontally orvertically adjacent to the guard area.

FIG. 17 b is a schematic illustration of a second mode of operation of aguard band. This mode of operation may apply to either a single display,or to a tiled display. Here, the information to be displayed by thepixels 540 a, b, c, d and e in a guard band 180 e matches theinformation to be displayed by the closest pixels 530 a, b or c in anactive area 170 e around which the guard band 180 e is formed. Forinstance, the information used to drive the pixel 530 a, located on oneedge of the active area 170 e will also be used to drive theneighbouring pixel 540 a. Similarly, the information used to drive thepixel 530 b, located on another edge of the active area 170 e will alsobe used to drive the neighbouring pixel 540 b. In the case of the pixel530 c, located on a corner of the active area 170 e, the informationdriving this pixel will also be used to drive the guard band pixels 540c, d and e.

Experimentation has shown that where pixels in the guard band 180display the same colour and luminance as the nearest adjacent pixels inthe active pixel region 170, this provides preferable visual imageproperties for text/graphics-based image data (here, the term“text/graphics” is used to signify hand-drawn or computer-generatedmaterial (graphs, letters etc) which have the characteristic of rapidcontrast and/or hue changes (in the spatial domain), whereas otherimages (photographs, paintings etc) have (generally) more gradualspatial changes of contrast and/or hue. On the other hand, where thepixels in the guard band 180 display the same colour and luminance asthe nearest adjacent pixels in the next adjacent display, this providespreferable visual image properties for non-text/graphics image data.

A single image frame to be displayed over an array of tiled displays mayinclude regions comprising text/graphics and regions withouttext/graphics. In this case, it is desirable that guard band pixelsfalling within regions comprising text/graphics should be written toduplicate the colour and luminance of the nearest adjacent pixels in theactive region 170 and that guard band pixels falling within regionswithout text/graphics (or with only small amounts of text/graphics)should be written to duplicate the colour and luminance of the nearestadjacent pixels in the next adjacent display.

Both the guard bands, and the regions of the active areas used to drivethem, are not necessarily limited to a single width of pixels, but mayinclude a plurality of rows and columns.

Preferably, the basis on which the colour and luminance of the guardband pixels is determined (i.e. the mode of operation) is selectable. Inparticular, the mode of operation may be either auto-selectable bycontrol circuitry in the display device, or manually selectable by auser.

The display may include an electronic detector operable to detect thetype of information being displayed (e.g. text/graphic or image only)and to write the guard band pixels appropriately. More specifically, thetype of information displayed at each inter-tile boundary can beanalysed, and the guard band of each tile can be written accordingly.The electronic detector may be included in the control circuitry drivingthe display. An example display controller operable to detect andisolate text/graphic regions from other image regions is the PhillipsSAA6713 display controller.

FIG. 18 schematically illustrates example control circuitry which may beused to drive a tiled array of displays. The control circuitry receivesimage data 600 representing the image to be displayed on the tiled arrayof displays. The image data 600 is passed to a demultiplexer 610 and atile edge region detector 620. The demultiplexer 610 is arranged toseparate the incoming image data 600 into image data representative ofthe image to be displayed at individual tiles of the tiled display. Theseparated image data is passed to the appropriate respective individualtiles 650 a, b and c. The tile edge region detector 620 is arranged todetect parts of the incoming image data 600 which are close to aninter-tile boundary. The edge regions do not necessarily need tocorrespond to the areas of further pixels, but can extend beyond this,potentially to include the entire tile. The image data categorised bythe tile edge region detector 620 as being at a tile edge is passed toan image type detector 630. The image type detector 630 detects whetherthe image data relates to (for instance) text/graphics only data or datacomprising image, the result of this detection being used to generate acontrol signal 640 to be passed to the demultiplexer. The demultiplexer610 includes information regarding the guard pixels in the image data tobe sent to the tiles 650 a, b and c, in response to the control signal640.

FIG. 8 is a schematic side elevation of an array of light transmissionguides 80′″ coupled to pixel elements 100′″ on an image output surface60′″. The coupling is such that lateral movement between the lighttransmission guides 80′″ and the image output surface 60′″ is notcompletely inhibited.

In this example arrangement, each light transmission guide 80′″ receiveslight from a rectangular array of 4×4 pixel elements 100′″. Thearrangement shown in FIG. 7 illustrates the light transmission guidesexactly covering the active pixel region 170 of the image output surface60′″. However, if any expansion occurs in which the image guide expandsto a greater extent than the panel display, the image guide will tend tomove outwards in a direction 190. This will bring the outer most lighttransmission guide over pixels in the guard band 180.

FIG. 9 is a schematic plan view of the arrangement of FIG. 8, showingone corner of the image output surface 60′″ including the guard band 180which is a row of pixels two pixels wide running around the active pixelarea 170.

FIG. 10 is a schematic plan view of an array of light transmissionguides 80″″ coupled to an image output surface 60″″ in accordance with afourth embodiment of the present invention.

FIG. 11 is a schematic side elevation of the arrangement of FIG. 10, inwhich the spacing between the light transmission guides 80″″ has beenexaggerated for clarity.

Each light transmission guide 80″″ overlies a respective pixel element,although other arrangements as described above are of course possible.The light transmission guides 80″″ are separated by gaps 200 which arefilled with a compressible material such as an open cell foam adhesive.

Surrounding the whole array of light transmission guides 80′″ at theirinput end, is a rigid frame 210 formed of a material havingsubstantially identical thermal expansion properties to that of theimage output surface 60″″. Generally, this will be a glass material, butit has been found that metals may also be used as they have thermalexpansion properties which are much closer to those of glass than tothose of plastics or polymers.

For some display types, such as LC displays, the image output surface60″″ may include a layer formed of a material having different thermalexpansion properties to the frame 210. For instance, with LC displays,the image output surface 60″″ may have a polymeric film acting as apolariser adhered to it. In this case, where a polymeric polarising filmis adhered to an underlying structure (which may for instance be formedof glass), the expansion properties of the polarising film will remainsubstantially those of the underlying structure, with the polarisingfilm being “stretched” as the underlying structure expands (andcompressed as the underlying structure contracts). Although the frame210 may be coupled to the polarising film, because the expansion of thefilm will be determined by the thermal expansion properties of theunderlying structure, the thermal expansion properties of the frame 210should be substantially the same as those of the underlying structure.If some differential expansion occurs, the whole array of lighttransmission guides 80″″ cannot expand at their input end (in a lateraldirection) beyond the rigid frame 210. The frame in turn expands atsubstantially the same rate as the image output surface 60″″.

So, expansion at the input end of the light transmission guides is takenup by the compressible material in the gaps 200. Assuming that the gapsare relatively uniformly filled, this provides a uniform compression ofthe compressible material across the array of light transmission guides80″″. This in turn means that the alignment or registration between thelight transmission guides at their input end and pixel elements on thisimage output surface 60″″ is not compromised.

Preferably, the means by which the frame 210 is attached to the array oflight transmission guides 80″″ will conserve the optical characteristicsthat exist between non-peripheral fibres in the array. Embodiments ofthe invention which address this are illustrated in FIGS. 16 a and 16 b.There may be an inner frame 212 of a material having the substantiallysimilar optical characteristics to the light transmission guides 80″″and being coupled to the peripheral light transmission guides (i.e.those adjacent to the frame) in a manner that is optically substantiallyidentical to the manner in which the light transmission guides 80″″ areattached to each other. The inner frame 212 would be sandwiched betweenthe outer row of light transmission guides 80″″ and the outer frame 211(which would lend rigidity to the structure). The inner frame 212 couldfor instance take the form of a continuous boundary of materialsurrounding all sides of the array or could comprise an additionalboundary of light transmission guides 213 around the array. In thelatter case, the additional light transmission guides 213 would betruncated so as not to reach the plane of the output ends of the lighttransmission guides 80″″. The outer frame 211 may have the same orsimilar optical characteristics to the light transmission guides 80″″,but different thermal expansion characteristics.

Preferably, light entering into the inner frame 212 will be prevented orat least inhibited from exiting the inner frame 212 in a manner thatwould degrade the visual properties of the display. For instance, wherethe inner frame 212 comprises truncated light transmission guides 213,these could have their truncated ends covered (e.g. coated) with a lightabsorbing layer. Alternatively, where the inner frame 212 comprises acontinuous boundary of material, the part of its surface not in contactwith the peripheral light transmission guides 80 could be covered (e.g.coated) with a light absorbing layer.

FIG. 12 a schematically illustrates a side elevation view of a typicalarrangement as described above and FIG. 12 b schematically illustrates aplan view of the image output surface 60 of the same arrangement. InFIG. 12 a, a single light guide 20 is coupled to an active region 170 ofan image output surface 60. FIG. 12 b shows that the input ends of lighttransmission guides 80 making up the light guide 20 are arranged toreceive light from an active area 170 of the image output surface 60.

With this arrangement, each light guide 20 may be formed of an array ofindividual light transmission guides 80 (channels) that are close-packedas a regular array (typically square packed) at input and output withthin layers of glue between the light transmission guides. This packingconfiguration means that the array behaves more as a single large area(i.e. a continuous sheet) than as a group of individual lighttransmission guides (in terms of expansion) and the expansion in theplane parallel to the input apertures of the light transmission guides80 and the image output surface 60 is cumulative. For instance, if eachinput aperture expands by 1% of its linear dimension then the linearexpansion of the inputs of a close packed line of ten light transmissionguides 80 will be approximately 10% of the linear dimension of a singlelight transmission guide 80.

For a large number of light transmission guides 80, the cumulativeexpansion of the input face of the light guide 20 relative to the lowerexpansion of the image output surface 60 (such as the modulator plane ofa LC panel) may cause serious loss of registration if a large imageoutput surface 60 (e.g. large area modulator of LC display) is used.

In an effort to contain the effect of the relative motion of the inputends of light transmission guides 80 relative to the pixel elements ofthe image output surface 60, the accumulation of the error should belimited. One method of addressing this problem is to limit the lineardimension of the input surface of the light guide 20. However, reducingthe dimensions of the input surface of the light guide 20 necessitatesusing a larger number of reduced dimension output image surfaces (e.g.small modulating arrays) rather than a smaller number of regular imageoutput surfaces (e.g. standard or large modulating arrays). It isdesirable to use fewer image output surfaces 60 to reduce cost, improveefficiency and aid manufacturing.

FIGS. 12 c and 12 d schematically illustrate an alternative embodimentof the invention in which the input face of a light guide 20 a is splitinto a plurality of groups 25 a, b, c, d of light transmission guides 80while a continuous array is maintained at the output face of the lightguide 20 a. FIG. 12 d shows how the image output surface 60 b comprisesa plurality of active regions 170 a, b, c and d, separated from eachother by non-active regions 185, in this case a cruciform shape of“dead” or boundary pixels. Each group 25 a, b, c, d of lighttransmission guides 80 is coupled to a different one of the activeregions 170 a, b, c or d and receives light from pixel elements in thatactive region. Fabricating a light guide 20 a in this way, to have aplurality of (e.g. four) segments enables expansion to take placeinwards (into the non-active regions 185 between the segments) as wellas outwards, therefore reducing the degree of pixel-light transmissionguide misregistration at the edges of the light guide 20 a. For an inputend which is divided into four sections, the linear dimensions of eachsection will be half the linear dimension of a continuous input end,minus half the desired dimension of the non-active area 185. For an Ngroup by N group division, the degree of misaligmnent resulting fromthermal expansion can be reduced to 1/N of that expected for acontinuous input end. Additionally, it may be advantageous in helping tofill and index match the near planar area between the light guide inputand the image output display surface (e.g. LC panel) to have foursmaller areas rather than one large area, due to the difficultiesinvolved in evenly applying index matching gel between two large-areaplanes.

It is possible to provide this arrangement with very little difference(or no difference at all) in the moulding (assuming for instance, thatfor a four group light guide, one quarter of a single row would usuallybe moulded as a single unit in the single group light guide embodiment).

In one embodiment, the displacement towards the centre (to cover theunused pixels of the image output surface 60 a) is moulded into thequarter row (or whatever fraction of a row is moulded at one time) suchthat when the complete rows are assembled the output of the light guide20 a is continuous and the input of the light guide 20 a has a cruciformgap that matches the active regions/modulation areas 170. The width ofthe gaps between the active areas of the image output surface 60 a canbe either small (e.g. 2 pixels=0.615 mm for 0.3075 mm pixels) or forwider (e.g. 20 pixels=6.15 mm for 0.3075 mm pixels).

The former (small width of unused pixels) is advantageous in onerespect, because large areas of unused pixels on an image output surface60 a are wasteful and inefficient. A further advantage of having a gapat the centre of the image output surface 60 a is that the expansion ofthe light guide 20 a can take place inwards as well as outwards therebyreducing the loss of registration by a factor of approximately 2.

Where there is a very small gap (i.e. small number of unused pixels)between the active areas 170 on the image output surface 60 a, it is notstrictly necessary to mould the inward bend into the quarter (or otherrelevant fraction) row. Instead, because the displacement is small andthe resulting stresses would therefore also be small, the outputapertures of the four quarters could be pushed together and glued duringthe assembly process, resulting in a more straightforward manufacturingprocess.

The latter (wider width of unused pixels) has a different advantage inthat it allows the possible addition of a registration aid between therespective groups of input ends of the light guide 20 a. Theregistration aid may be attached to the image output surface 60 a of thedisplay device. FIG. 12 e schematically illustrates a displayarrangement comprising a registration aid 300. The registration aid 300could take the form of a cruciform frame attached to the image outputsurface 60 a (e.g. a metal or glass frame fixed to the output glass ofan LC panel) such that the input channels can be butted against it as ameans of registering it or even of securing it. Further possible formsinclude unconnected bars (illustrated schematically in FIG. 13 a),connected or interlocking bars, and ‘L’ shaped objects to locate cornersof the input end of the light guide 20 a (illustrated schematically inFIGS. 13 b and 13 c). Expansion would be outwards from the points ofcontact between the light guide 20 a and the registration aid 300.Preferably, the registration aid 300 would be made of glass having thesame or similar thermal expansion properties to the image output surface60 a. Registration aids can also be provided in non-active areasexternal to the regions between the active areas.

A further embodiment of the invention, illustrated schematically in FIG.13 d provides a frame 210 a which constrains the periphery of the inputface of the light guide 20 a The input face would be “shoehorned” intothe frame 210 a on assembly of the unit. The lack of adhesive betweenthe sections allows expansion to occur inwards from the frame 210 a.

In the case of, for instance, an LC display, the image output surface 60may have a different (e.g. lower) coefficient of thermal expansion (CTE)than a light guide 20 coupled to it. If the image output surface 60 andthe light guide 20 are attached together by means of a rigid, orsemi-rigid adhesive, the adhesive layer will become stressed inconditions where the temperature varies from the temperature at whichthe attachment was made. Consequently, mechanical distortion of thelight guide 20 and/or the image output surface 60 (or the LC displayitself) may arise. This mechanical distortion may adversely affect theperformance of the display, and may ultimately lead to failure of theadhesive bond and thus to the decoupling of the image output surface 60and the light guide 20. On the other hand, if the adhesive layer used iselastically compliant, then misregistration between the pixels of theactive area 170 of the display 60 and the light transmission guides 80of the light guide 20 may occur.

An alternative method of attaching a light guide to an image outputsurface is schematically illustrated in FIG. 14. In this arrangement,instead of using a continuous layer of adhesive, an adhesive 400 isconfined to a small area, the dimensions of which should be sufficientto support its share of the display mass attached to a light guide 20 b.The adhesive 400 is disposed centrally in the integral area of the lightguide 20 b. The remainder of the area between the light guide 20 b andan image output surface 60 c that is required to be optically coupled isfilled with a gel having an extremely high viscosity or an elastomericresin having an extremely low modulus of elasticity such that expansiondifferences produce no significant stress within the layer, and havingsubstantially the same refractive index as the adhesive 400.

A suitable combination of adhesive 400 and low modulus elastomer wouldbe Dymax X413-25-A (refractive index, n=1.42) as the adhesive 400 andDow Corning 787T (refractive index, n=1.428) as the elastomer. Othersuitable adhesives include Norland NOA81 (refractive index, n=1.56),Dymax OP4-20655 (refractive index, n=1.48) and Dymax OP4-20641(refractive index, n=1.505). These could be used with gels such asLS-3238 Curing Encapsulation Gel (refractive index, n=1.38), LS-3246(refractive index, n=1.46), LS-3249 (refractive index, n=1.49), LS-3252(refractive index, n=1.52) and LS-3357 (refractive Index, n=1.57). Theseadhesives may be advantageous over the lower refractive indexcombination above, having refractive indices between those of apolarising layer present on the image output surface, and of the lighttransmission guides 80, where the light transmission guides 80 areformed of polycarbonate.

In the embodiment of FIG. 14, the adhesive 400 alone may not providesufficient torsional rigidity to prevent misregistration throughrotation of the light guide 20 b and the image output surface 60 c withrespect to each other. To overcome this, two (or more) reference points410 are glued rigidly to the image output surface 60 c. The referencepoints 410 comprise low modulus glue that will yield to expansion forcesresulting from variations in temperature.

With this arrangement, strain is allowed to occur at minimum stress byusing adhesive over only a small area of the interface between the imageoutput surface 60 c and the light guide 20 b. This advantageouslyprovides that the area of rigid or semi-rigid adhesive sufficientlysmall to reduce stresses within the adhesive layer and that anyexpansion will take place symmetrically from the centre of the display,halving the misalignment compared to a method that fixes alignment fromone corner of the display.

FIG. 15 schematically illustrates another embodiment which uses a smallarea of adhesive 400 to fix a light guide 20 c to an image outputsurface 60 d. In FIG. 15, the light guide 20 c is split into a pluralityof groups of light transmission guides 80 (in this case, four groups) asdescribed above with reference to FIG. 12, in an arrangement otherwisesimilar to that described with reference to FIG. 14. Here, wheremultiple input segments of the light guide 20 c are used, registrationpoints 410 on the image output surface are not required, sincesufficient torsional rigidity can be provided by the multiple adhesiveareas 400.

Although the above embodiments of the invention are described such thatthe light transmission guides 80 of the light guide 20 are attacheddirectly to the image output surface 60 of the display device, the endsof the light transmission guides 80 could also be attached to asubstrate having substantially the same expansion properties as thedisplay device. The substrate could be removably coupled to the imageoutput surface 60 of the display device.

This arrangement might be more desirable than attaching the lighttransmission guides 80 directly to the display device (for yield, costand maintenance reasons). The ends of the light transmission guides 80could be glued directly to a substrate by applying glue to the ends ofthe light transmission guides 80 or to the substrate, placing the lighttransmission guides 80 row by row accurately in position using a linearstage or stages and holding the light transmission guides 80 in positionwhilst the glue is cured. This would avoid the need for the blades 140described in relation to FIG. 6.

1. An image display comprising: a display device having an image outputsurface at which an image is displayed as a spaced array of pixelelements over an active pixel region of the image output surface; animage guide coupled to the active pixel region of the image outputsurface and comprising a plurality of light transmission guides eachhaving an input end and an output end, the input ends of the lighttransmission guides being arranged relative to one another so thatgroups of one or more light transmission guides receive light fromrespective groups of one or more pixel elements; in which: the imageoutput surface has further pixel elements disposed around the peripheryof the active pixel region; and the image guide can expand thermally sothat the input of the image guide encompasses the further pixels.
 2. Theimage display according to claim 1, in which the image output surfacehas further pixel elements disposed around the periphery of the activepixel region over a guard band region narrower than the input end of alight transmission guide.
 3. The image display according to claim 1, inwhich each light transmission guide is arranged to receive light fromtwo or more pixel elements.
 4. The image display according to claim 1,in which the further pixels are arranged to display substantially thesame picture information as nearby pixels within the active pixelregion.
 5. The image display according to claim 1, wherein the displayincludes: a tiled array of displays arranged so that viewing surfacesformed by the output ends of the image guides abut to form a largercomposite viewing surface; in which the further pixels are arranged todisplay substantially the same picture information as pixels within theactive pixel region of an adjacent display in the tiled array which areadjacent to the further pixels in the composite viewing surface.
 6. Theimage display according to claim 1, wherein the display includes: atiled array of displays arranged so that viewing surfaces formed by theoutput ends of the image guides abut to form a larger composite viewingsurface; in which the further pixels are selectably operable to displayeither substantially the same picture information as pixels within theactive pixel region of an adjacent display in the tiled array which areadjacent to the further pixels in the composite viewing surface orsubstantially the same picture information as nearby pixels within theactive pixel region of the same display.
 7. The image display accordingto claim 6, wherein the picture information displayed by the furtherpixels is user selectable.
 8. The image display according to claim 6,wherein the picture information displayed by the further pixels isautomatically selected on the basis of predetermined criteria.
 9. Theimage display according to claim 8, further comprising a detector fordetecting the type of information being displayed by the nearby pixelsand selecting the picture information displayed by the further pixels onthe basis of the detection result.
 10. The image display according toclaim 9, wherein if the detector detects information having at least athreshold rate of change of contrast and/or hue, the picture informationdisplayed by the further pixels will be selected to be substantially thesame picture information as nearby pixels within the active pixel regionof the same display.
 11. The image display according to claim 9, whereinif the detector detects information having less than the threshold rateof change of contrast and/or hue, the picture information displayed bythe further pixels will be selected to be substantially the same pictureinformation as pixels within the active pixel region of an adjacentdisplay in the tiled array which are adjacent to the further pixels inthe composite viewing surface.